Using "Univ" / "=.." meta-predicate to return boolean result of a function?

I am trying to use a meta-predicate "=.." to return a value.

Now I have two examples below, the first one works perfectly, however I cannot figure out how to do something similar using the second example:

Example 1:

?- Expression=..[+,2,3], Value is Expres开发者_如何转开发sion.
Expression = 2 + 3
Value = 5
yes

Example 2:

?- test(N):-N>=0.
Term asserted

?- Term=..[test,-5], Value is Term.

This is producing an error, and i want Value to be yes or true or 1, so that I can use it in an IF statement.

How can this be accomplished using the "univ" "=.." operators?

You can't use is/2 to get the boolean value of a predicate call. Use call/1 instead:

?- Term =.. [test, -5], call(Term).
false.
?- Term =.. [test, 5], call(Term).
Term = test(5).

Or just:

?- call(test, 5).
true.

?- call(test, -5).
false.

Then your if-then-else just becomes:

(Term =.. [test, -5], call(Term) ->
    % then-part
;
    % else-part
)

If you want to reify the result value for some other reason, the usual advice applies:

(Term =.. [test, -5], call(Term) ->
    Value = true   % or 1
;
    Value = false  % or 0, or fail
)